A technology that promises to bring all the advantages of spatial multiplexing to point-to-point communications is MIMO. It is advised as one of the most advanced technologies in wireless communications up to date. The MIMO system is a com- munication system using multiple antennas on both sides of communications. It is taking an advantage of the multipath propagation, synthesizing a number of indepen- dent communication sub-channels by means of a proper handling of the signals at the transmitting and at the receiving antennas. An angular diversity in Space Division Multiple Access (SDMA) requires an angular separation of the users which is provided by the multiple paths where the electromagnetic waves follow from the transmitting array to the receiving one.
The earliest concepts regarding the capacity of MIMO communication systems were published in the seventies by Kaye and George [161] and Van Etten [162]. In 1987 J.H Winters [163], had proposed MIMO systems. He investigated the data rates of multiple antenna systems. In 1994, Paulraj and Kailath patented a system employing a “distributed transmission and a directional reception” [164]. Finally, in 1998, Foschini and Gans had stated the limits on the performances of MIMO systems [165]. A prototype of MIMO systems was first demonstrated at Bell Laboratories. Recently, theoretical and experimental published articles confirm the superior data rate, multipath fading reduction and co-channel interferences suppression. Moreover, nowadays, the trend for mobile terminals is to accommodate the increased number of wireless communication applications.
A printed diversity monopole antenna for 2.4 GHz (WLAN) band is investigated by T.Y. Wu [166]. Here, two orthogonal linear monopole antennas are placed sym- metrically between T-shaped ground plane. This antenna has the isolation (S21) ≤
-20 dB. It is capable of combating the multipath interference problem for WLAN operation.
rically printed on the two sides of a T-shaped protruded ground plane. It is capable of operating in a very wide bandwidth, defined by a -10dB reflection coefficient, of about 108 % (about 2.3-7.7 GHz). Across the operating bandwidth, the antenna also shows good port isolation (S21≤-20 dB) and can provide spatial diversity to combat
the multipath fading problem. To compute the envelope correlation of an antenna diversity system, a simple formulation is derived by S. Blanch et al. [168]. Here, the envelope correlation is computed from the S-parameter description. This approach does not require the computation or the measurement of the radiation pattern of the antenna system. This technique provides a clear understanding of the effects of isolation and input match on the diversity performance of the antenna system.
M.A. Jensen et al. [169] have described a review for MIMO wireless communica- tions systems. They have considered issues including channel capacity computation, channel measurement & modeling approaches, and the impact of antenna element properties for array configuration on system performance.
G. Chi et al. [170] have designed dual-band printed diversity antenna for WLAN 2.4/5.2 GHz operations. Here, two orthogonal C-shaped monopoles are placed sym- metrically with respect to a protruding T-shaped ground plane. The antenna has the isolation (S21)≤-25 dB. The proposed antenna is capable of combating the multipath
interference problem for WLAN operation.
A simple closed-form equation to calculate the envelope correlation between any two receiving or transmitting antennas in a MIMO system of an arbitrary number of elements is derived by J.Thaysen et al. [171]. The equation uses the scattering parameters obtained at the antenna feed point to calculate the envelope correlation coefficient.
A dual-band printed diversity antenna is designed by Y. Ding et al. [172]. This antenna consists of two back-to-back monopoles with symmetric configuration. A prototype of the antenna operated at UMTS (1920-2170 MHz) and 2.4-GHz WLAN (2400-2484 MHz) bands is provided to demonstrate the usability of the methodology in dual-band diversity antenna for mobile terminals. The measured bandwidths are 1.86-2.19 GHz and 2.40-2.49 GHz with acceptable isolation over the bandwidths.
S. Hong et al. [173] have designed a two-element diversity planar antenna for MIMO applications. This antenna provides wideband impedance matching charac- teristics over the desired frequency band by adopting two Y-shaped radiators. In order to reduce the mutual coupling between two radiating elements, three stubs are inserted in the ground plane. Its measured impedance bandwidth over 2.27-10.2 GHz for a reflection coefficient of less than -10dB is obtained.
A quad-band monopole antenna array for MIMO enabled wireless communication is presented by R.A. Bhatti et al. [174]. Here, a C-shaped slot and a T-shaped slit are used to excite three current modes for proposed antenna. It exhibits quite low isolation coefficient and reflection coefficient at the desired four frequency bands. The proposed dual-element antenna array operates at the following frequency bands: 2.4- 2.5 GHz, 3.4-3.6 GHz, 5.15-5.35 GHz, and 5.75-5.875 GHz.
S. Zhang. et al. [175] have proposed a compact printed UWB MIMO/diversity antenna system (of two elements) with a size of 35×40 mm2
operating at a frequency range of 3.1-10.6 GHz. The wideband isolation can be achieved through a tree-like structure on the ground plane.
S.W. Su [176] has proposed a high-gain, three-antenna systems suitable to be con- cealed inside wireless access points for MIMO applications in the WLAN 2.4/5.2/5.8 GHz bands. Here, the port isolation can be obtained together with high-gain and directional radiation characteristics. The calculated envelope correlation for this pro- posed antenna is less than 0.007 within the bands of interest.
A compact dual-band MIMO antenna with high port isolation is proposed by S. Cui et al. [177]. This antenna is basically composed of two folded monopoles and is designed to operate at 2.4/5.6 GHz frequency band. The high isolation is achieved by introducing two transmission lines on the top surface of the substrate and etching two slots on the ground.
J.F. Li et al. [56] have designed a compact wideband MIMO antenna. Here, two symmetric monopoles with edge-to-edge separation of nearly 0.083λ0 at 2.5 GHz and
two bent slits are etched out from the ground plane. At the lower frequencies, the bent slits are used for reducing the mutual coupling and have slight effect on the
reflection coefficient. At the higher frequencies, the slits are used as slit antennas to enhance the impedance bandwidth because the two slits are couple-fed with two 50Ω microstrip lines.
Tri-band four-element MIMO antenna with high isolation is presented by J.F. Li et al. [178]. It consists of four symmetrical antenna elements. In this study, four rectangles have cut from the four corners of the ground plane to improve the operational bandwidth of the antenna. In addition, two kinds of isolation structures are introduced to reduce the isolation for the four antenna elements.
C.H. See et al. [179] have proposed a novel printed diversity monopole antenna for WiFi/WiMAX applications. This antenna comprises two crescent shaped radiators placed symmetrically with respect to a defected ground plane. A neutralization line is connected between them to achieve good impedance matching and low mutual coupling. Theoretical and experimental characteristics of this antenna exhibit an impedance bandwidth of 54.5% (over 2.4-4.2 GHz) with a reflection coefficient < -10 dB and mutual coupling < -17 dB.
A compact printed MIMO antenna for tetra band (GSM900/1800/1900/UMTS) mobile handset application is presented by Q. Zeng et al. [180]. It consists of two couple-fed loop antennas with symmetrical configuration. The edge-to-edge spacing between the two elements is only 0.03λ0 of 920 MHz. A slot and a dual inverted L-
shaped ground branch are added in the ground plane to decrease the mutual coupling between the antenna elements. The measured isolation of the proposed antenna is better than -15dB among the four operating frequency bands.
Recently, frequency reconfigurable microstrip antenna operating at Digital TV and LTE bands with MIMO implementation is designed. This antenna is matched from 496-862 MHz (DTV) and LTE bands 3 & 7 from 1710-1880 MHz and 2500-2700 MHz, respectively for all considering S11=-6dB matching criterion. The proposed antenna